We examined whether adenosine equally attenuated the stimulatory effects of isoproterenol on arrhythmic activity and twitch shortening of guinea pig isolated ventricular myocytes. Transmembrane voltages and whole cell currents were recorded with patch electrodes, and cell twitch shortening was measured using a video-motion detector. Isoproterenol increased the action potential duration at 50% repolarization (APD₅₀), L-type Ca²⁺ current [I_Ca(L)], and cell twitch shortening and induced delayed afterdepolarizations (DAD), transient inward current (I_Ti), and aftercontractions. Adenosine attenuated the arrhythmogenic actions of isoproterenol more than it attenuated the effects of isoproterenol on APD₅₀, I_Ca(L), or twitch shortening. Adenosine (0.1-100 µmol/l) decreased the amplitude of DADs by 30 ± 6% to 92 ± 5% but attenuated isoproterenol-induced prolongation of the APD₅₀ by only 14 ± 4% to 59 ± 4% and had no effect on the voltage of action potential plateau. Adenosine (30 µmol/l) inhibited I_Ti by 91 ± 4% but decreased isoproterenol-stimulated I_Ca(L) by only 30 ± 12%. Isoproterenol-induced aftercontractions were abolished by adenosine (10 µmol/l), whereas the amplitude of twitch shortening was not reduced. The effects of adenosine on twitch shortenings and aftercontractions were mimicked by the A₁-adenosine receptor agonist CPA (N⁶-cyclopentyladenosine) and by ryanodine. In conclusion, adenosine antagonized the proarrhythmic effect of -adrenergic stimulation on ventricular myocytes without reducing cell twitch shortening.